Remove TreatNullAs=NullString for HTMLFormElement

http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#htmlformelement

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

Firefox reflects autocomplete as 'on', which is per spec since
autocomplete is an enumerated attribute, but this will not be fixed in
Blink as part of this change.

IE11 throws an exception for enctype, encoding and method.

Opera doesn't support autocomplete and reflects encoding as 'null'.

BUG=310298

Review URL: https://codereview.chromium.org/60733023

git-svn-id: svn://svn.chromium.org/blink/trunk@162029 bbb929c8-8fbe-4397-9dbb-9b2b20218538